Long distance optical fiber transmission systems which use inexpensive, broad band, erbium doped optical fiber amplifiers rather than costly electronic regenerators are now under development for terrestrial and undersea applications. The distance that can be spanned with an optical fiber before regeneration of the optical signal becomes necessary is determined by the loss in the fiber and by its dispersion characteristics. Fiber losses in commercially available optical fiber have been reduced to about 0.2 dB/km at 1.55 .mu.m wavelength. Thus, if a receiver can detect signals that are 20 dB below the input signal, a distance of 100 km can be spanned before signal amplification is required. Once the repeater span is determined, the maximum signaling rate depends on dispersion of the signal within the fiber. With a monochromatic light source, data rates on the order of 100 Gb/sec are technically feasible if the fiber is operated at the zero-dispersion wavelength.
By inserting optical amplifiers into the optical fiber cable, the range over which a signal can be detected can be extended. Dispersion, however, will still set an upper limit on the maximum spacing between the amplifiers if high data rates are desired.
But, it is now known that optical fibers have a small amount of nonlinearity which enables certain special pulse shapes to travel long distances without changing shape. Signals which have this special waveform are called solitons. Solitons carry a given amount of light energy that is related to the duration of the pulse, the fiber nonlinearity and the fiber dispersion. Thus, with solitons, the upper limit on the maximum spacing between amplifiers can be increased substantially.
One structure for generating solitons consists of a high speed amplitude modulator coupled to receive shaped electrical signals for modulating an optical signal. The shaped electrical signals are formed by combining a multitude of individual signals each having a frequency normally greater than 2 MHz. The process of combining the various high frequency signals into a single signal results in relatively large signal loss and a need for signal amplification. But, the transmission and processing of the final signal, which is a composite of many high frequency signals, is extremely difficult. For example, a conductor, even for lengths as short as 1 cm, can change the shape of the multi octave high frequency composite signal. Thus, the composite signal received by the modulator can have a waveshape which is different from the waveshape of the signal which is generated. Moreover, if the composite signal requires amplification, then a relatively expensive amplifier capable of uniformly amplifying a composite signal of many very high frequencies is required. This invention is directed towards an improved soliton generator which avoids the above noted problems.